No not only portrait, working as landscape on my Android tablet. I think the app detects the device and either applies the portrait, aka phone mode, or landscape/tablet mode. There is no user setting as far as I know.
I own the remix computer and I’ve used Android Roon on it with no issues. So I would imagine the OS version would work just as well. I’m planning on putting on a couple of old computers to test it out.
Been a while since I looked at the Remix ROM, but if it has a Setting to adjust the DPI, then adjust that setting
The Roon app will look at the DPI setting and use that to decide what version of the App to run…have a look in the Pine64 rom to see if that setting exists
If either of them doesn’t have it then I know that CyanogenMod 12 and 13 [perhaps prior as well] have an adjustable DPI setting within Android Settings